Re: [PATCH v2 net-next] net: hsr: fix suspicious usage in hsr_node_get_first()

On 2/10/22 09:47, Eric Dumazet wrote:

On 2/10/22 08:23, Juhee Kang wrote:

quoted

Currently, to dereference hlist_node which is result of 
hlist_first_rcu(),
rcu_dereference() is used. But, suspicious RCU warnings occur because
the caller doesn't acquire RCU. So it was solved by adding 
rcu_read_lock().

The kernel test robot reports:
     [   53.750001][ T3597] =============================
     [   53.754849][ T3597] WARNING: suspicious RCU usage
     [   53.759833][ T3597] 5.17.0-rc2-syzkaller-00903-g45230829827b 
#0 Not tainted
     [   53.766947][ T3597] -----------------------------
     [   53.771840][ T3597] net/hsr/hsr_framereg.c:34 suspicious 
rcu_dereference_check() usage!
     [   53.780129][ T3597] other info that might help us debug this:
     [   53.790594][ T3597] rcu_scheduler_active = 2, debug_locks = 1
     [   53.798896][ T3597] 2 locks held by syz-executor.0/3597:

Please include whole stack.

quoted

Fixes: 4acc45db7115 ("net: hsr: use hlist_head instead of list_head 
for mac addresses")
Reported-by: Eric Dumazet <redacted>
Reported-by: syzbot+f0eb4f3876de066b128c@syzkaller.appspotmail.com
Signed-off-by: Juhee Kang <redacted>
---
v2:
  - rebase current net-next tree

  net/hsr/hsr_framereg.c | 3 +++
  1 file changed, 3 insertions(+)

diff --git a/net/hsr/hsr_framereg.c b/net/hsr/hsr_framereg.c
index b3c6ffa1894d..92abdf855327 100644
--- a/net/hsr/hsr_framereg.c
+++ b/net/hsr/hsr_framereg.c

@@ -31,7 +31,10 @@ struct hsr_node *hsr_node_get_first(struct 

hlist_head *head)
  {
      struct hlist_node *first;
  +    rcu_read_lock();
      first = rcu_dereference(hlist_first_rcu(head));
+    rcu_read_unlock();
+
      if (first)
          return hlist_entry(first, struct hsr_node, mac_list);

This is not fixing anything, just silence the warning.

I suggest replacing rcu_dereference() by rcu_dereference_rtnl()